Since Mark spec'd 91+ octane with his chip thats all I've run in my car since installing the chip a couple years ago. I let my wife borrow the car the other day & she forgot & filled up with 89 octane. I expected to hear pinging & experience a loss of power but so far there is no noticeable difference. All our gas is a 10% ethanol blend & I have read that you get a higher octane with the blend then what's stated on the pump. Maybe our 89 is actually 90+?? No idea but I've been driving around for several days now and the car runs fine. If there is a power loss its negligible and I haven't noticed it. With the price of gas these days I'm going to run the 89 for a couple tankfuls & see if there are any issues.